The use of sterically hindered aromatic diamines in the production of molded microcellular polyurethanes and molded polyurethane foams is known (German Offenlegungsschrift No. 2,622,951, U.S. Pat. Nos. 3,583,926, 4,218,543 and 4,374,222). By contrast, the present invention relates to the use of sterically hindered aromatic diamines in the production of free-rise polyurethane foams, particularly in the production of so-called spray foams (i.e., on site foam produced by spraying, cf. DIN 18159, Part 1, Section 3.2). Tough, rigid polyurethane foams, particularly spray foams, are obtained in accordance with the invention, which foams satisfy the requirements of Group B2 building materials as laid down in DIN 4102 and show firm adhesion to the surface.
Spray polyurethane foams which satisfy the requirements of Group B2 building materials as laid down in DIN 4102 are known as insulating materials in the building industry. In the spraying process, a highly active reaction mixture (in finely dispersed form under air or liquid pressure) is sprayed through the nozzles of a mixhead onto a surface that is to be insulated, where the mixture immediately begins to foam and hardens as a foam. The processing of spray foams, on site, is very seriously affected by inclement weather, resulting in significant economic losses. One of the most serious disadvantages of known spray foam systems lies in the fact that they can only be used at ambient temperatures above 10.degree. C. The effect of excessively low temperatures of the surface to be insulated is that too much heat of reaction is transferred from the first layer of the foamable polyurethane reaction mixture sprayed on the surface. This results not only in an increased gross density, but also embrittlement of the foam through incomplete reaction which can leave the surface of the foam which contacts the substrate to be insulated with a "sandy" appearance. The brittleness of the bottom surface of the foam is the reason for loss of the favorable adhesion properties to the substrate which the foam system shows when processed on substrate materials which are at too low a temperature.
The object of the present invention is to provide a polyurethane foam system which may be used with advantage, in particular by the spray process, and which avoids the disadvantages described above. It has surprisingly been found that it is possible to obviate the above-mentioned disadvantages by using certain sterically hindered aromatic diamines in the production of polyurethane foams, particularly by the spray process, i.e., in particular in the production of sprayed polyurethane foams. The sprayed foams obtainable in accordance with the invention surprisingly show firm adhesion to the substrate at temperatures as low as about 5.degree. C. without any deterioration in their burning behavior and other properties.